1. Field of the Invention
The present invention relates to a fixing unit and more particularly to a paper-separating mechanism for use in the fixing unit.
2. Description of the Related Art
With a conventional electrophotographic printer, a charging roller charges the surface of a photoconductive drum and an exposing unit such as an LED head writes an electrostatic latent image on the charged surface of the photoconductive drum. The electrostatic latent image is then developed with toner into a visible image, i.e., toner image. The toner image is subsequently transferred to a print medium when the print medium passes in a sandwiched relation between the photoconductive drum and a transfer roller.
FIG. 19 is a cross-sectional view of a conventional art fixing apparatus.
FIG. 20 is a top view illustrating a relevant portion of the conventional art paper-separating apparatus for use in the fixing apparatus.
Referring to FIG. 19, a fixing section 11 includes a heat roller 12 and a pressure roller 13. The heat roller 12 is rotatably supported and incorporates a halogen lamp 12a that functions as a heat-generating element. The pressure roller 13 is urged by urging means such as a spring, not shown, against the heat roller 12. The heat roller 12 and pressure roller 13 are accommodated in a casing 14. A paper-separating mechanism 80 is attached to the casing 14. The paper-separating mechanism 80 is rotatable with respect to the casing and separates recording paper from the heat roller 12 when the recording paper 15 has intimately wrapped around the heat roller 12 during the fixing operation.
The paper-separating mechanism 80 includes a substantially triangular lug 16 rotatably mounted on a shaft 81 and a guide roller 17 rotatably mounted on the lug 16. The guide roller 17 rotates freely on a shaft 83 and guides the recording paper 15 downstream therefrom. The lug 16 is urged by a compression spring 18 to rotate in a direction opposite to the direction shown by arrow B so that a tip 82 of the lug 16 is pressed against the heat roller 12. A paper-discharging mechanism 22 is provided downstream of the fixing section 11. The paper-discharging mechanism 22 includes a transport roller 19 and a pinch roller 20. The transport roller 19 is driven in rotation by a drive source, not shown. The pinch roller 20 is urged by a flat spring 21 against the transport roller 19, and rotates together with the transport roller 19.
The operation of the fixing apparatus of aforementioned construction will be described.
A toner image formed on the photoconductive drum, not shown, is transferred to the recording paper 15 by the transfer device. Then, as the photoconductive drum rotates, the recording paper having the toner image thereon is advanced substantially horizontally in a direction shown by arrow A. The recording paper 15 reaches the fixing section 11 and is pulled in between the heat roller 12 and pressure roller 13, so that the recording paper 15 is heated pressurized. As a result, the toner image is fixed and the printing completes.
As the heat roller 12 and pressure roller 13 rotate, the recording paper 15 having an image printed thereon is further advanced to the discharging section 22 where the recording paper is pulled in between the transport roller 19 and pinch roller 20 and discharged outside of the printer.
The operation of the paper-separating mechanism 80 will be described.
Prior to the arrival of the recording paper 15, the lug 16 is positioned at the dotted line position as shown in FIG. 19, i.e., a standby position where the lug 16 is in contact with the heat roller 12. When the leading end of the recording paper 15 comes into contact with the corner 16a of the lug 16, the recording paper 15 pushes the lug 16 up in a direction shown by arrow B against the urging force of the spring 18. As a result, the lug 16 moves to the solid line position of FIG. 19 where the tip 82 of the lug 16 is away from the heat roller 12. When the stopper 16b of the lug 16 abuts the casing 14 (i.e., solid line position), the stopper prevents further rotation of the lug 16 and the distance between the heat roller 12 and the tip 82 of the lug 16 reaches a maximum t.
Then, the transport roller 19 and pinch roller 20 advance the recording paper 15. When the trailing end of the recording paper 15 passes the corner 16a, the lug 16 returns to the dotted line position of FIG. 19 whether the lug 16 is in its standby state.
With the aforementioned conventional fixing apparatus, the corner 16a of the paper-separating lug 16 extends into a plane where the paper path lies. Therefore, when the recording paper 15 is advanced, part of the printed surface of the recording paper 15 rubs the corner 16a. When a solid black image is printed on the recording paper 15, the gloss of the image areas that rub the corner 16a is significantly different from that of the image areas that do not rub the corner 16a. This impairs the image quality.